Coloration of plastic resins is usually achieved using a masterbatch. A masterbatch is a granular, usually dust free colorant composition comprising substantially high concentrations of pigment. This masterbatch is normally added to a thermo-plastic resin, which is normally in granular form also, prior to processing of the mixture, by extrusion or injection for example.
The reason for using the masterbatch technique is that direct coloration with pigments raises issues due to poor dispersion. The masterbatch is also used for minimising the risk of abrasion of the extruders. Indeed, the pigments will tend to cover the surface of the processing apparatus without having a good dispersion within the resin itself.
The present invention relates to a process for coloration of a thermoplastic resin comprising the step of mixing the thermoplastic resin in a granular form with a colorant composition comprising pigments to form a granular mixture, the colorant composition being in the granular form during mixing.
Existing masterbatch or granular colorant compositions may comprise the pigments together with a carrier. The role of the carrier is to enhance the affinity of the pigment with the thermo-plastic resin for coloration and to improve the dispersion and mixing of the colorant composition with the resin.
Such compositions are disclosed in various documents. EP-A-0 751 171 published on the 2.sup.nd of Jan. 1997 describes a colorant composition whereby the carrier is comprising poly-olefins and poly-acrylate-ester. A different composition is proposed in U.S. Pat. No. 5,604,279 granted on the 18.sup.th of Feb. 1997, whereby the colorant composition also comprises poly-olefins together with a free flow agent. Such compositions should facilitate dispersion of the pigment within the final product. However, these colorant compositions are not compatible with all polymers, and for example have a poor affinity with polypropylene.
Consequently, it is an object of the present invention to provide a coloration process allowing better dispersion of pigments in a thermoplastic resin.